Seasonal and Flood-Induced Variations in Groundwater-Surface Water Exchange Dynamics in a Shallow Aquifer System: Amity Creek, MN

Excellent brook trout habitat can be found in segments of many streams on the North Shore of Lake Superior. Optimal riverine brook trout habitat includes clear, cold spring-fed water and studies have shown that areas of groundwater upwelling in streams tend to be more important than other site selection variables. Observations of historical brook trout and temperature data in the Amity Creek sub-watershed of Lake Superior South HUC-8 led us to hypothesize that reaches with healthy and stable brook trout communities are likely connected to groundwater storage, exchange, and upwelling. Climate trend models for the Midwest predict future changes in temperature, annual precipitation, and storm event frequency for Northern Minnesota. Streams along the North Shore of Lake Superior are susceptible to increased temperatures and insufficient late summer to early winter flows due to climate change and the unique regional geology. Understanding groundwater-surface water hydrology interactions, watershed connectivity, and related flood-induced geomorphic and hydrologic changes is important because they relate to the overall stability and aquatic health of the stream and the biological communities that inhabit it. The objectives of this study were to identify groundwater storage zones, upwelling zones and seasonal variations; and assess how these relationships change as result of a major flood. A study reach on East Branch Amity Creek is incising through clay-rich glacial tills overlying bedrock and has avulsed multiple times in the past, stranding discrete remnant channels cut into till. A 500-year storm hit Duluth, MN, USA on June 19-20th 2012, producing 8-inches of rain in a 24-hour duration which resulted in flash flooding across the region. Pre-flood and post-flood groundwater and surface water level data were collected through a series of piezometers with pressure transducers and an in-channel stream gage. Stable isotope analyses of Deuterium and Oxygen-18 were conducted on water samples with varying temporal and spatial variability to provide information on watershed and reach scale source hydrology and evaluate flood-induced changes. Results show groundwater upwelling variability within the greater watershed and identify two discrete groundwater storage zones within a smaller study reach. Pre- and post-flood analyses show a correlation between incision in main stem and remnant channels and a lowered water table, following the flood. Isotope analysis indicates a temporary post-flood change in subsurface source water. Because much of the greater North Shore of Lake Superior has the same geology as the study area, the results of this study may provide insight to hydrology studies of other North Shore streams.